Background/Purpose: TNF-alpha (TNFa) has been shown to contribute to osteoclastogenesis (OCgenesis) independently and in conjunction with M-CSF or RANKL, two key cytokines involved in osteoclast (OC) development. We have previously demonstrated that TNF enhances the kinetics of RANKL-induced human OCgenesis and that its effects are mitigated more effectively by the anti-TNF biologic adalimumab (ADA) as compared to etanercept (ETN). The objective of this study was to determine whether Fc-gamma receptor (FcgR)-mediated internalization of the biologic:TNF complexes is a contributing mechanism responsible for the difference in effectiveness between ADA and ETN in preventing TNF-enhanced OCgenesis.

Methods: TNF biologics [ADA and ETN] alone or in preformed complexes with TNFa at 50:1 molar ratio were tested for FcgR binding by flow cytometry using CHO stably transfected with human FCGRs (FcgRI, FcgRIIA, -RIIB, -RIIC, FcgRIIIA & -RIIIB). FcgR expression and binding of preformed biologic:TNF complexes at 10:1 ratio +/- FcgR blocking antibodies to primary human OC precursors (OCP) was evaluated by flow cytometry. FcgR-mediated internalization was assessed by monitoring a reduction in OC survival in response to preformed biologic:TNF complexes (25:1 ratio) bound with saporin (ZAP), a ribosome-inactivating toxin, as anti-human Fc IgG Fab conjugate +/- FcgR blocking antibodies.

Results: The binding study to CHO (human FcgRs) cell lines showed that monomeric ADA and ETN bind similarly to FcgR (highly on high affinity FcgR and loosely on low affinity FcgRs) while preformed biologics: TNF complexes bind differently. ADA:TNF complexes bind to low affinity FcgR, whereas ETN:TNF keep a monomeric binding profile with no gain of binding to low affinity FcgR. OCP were found to express mostly FcgRII early in development with predominant binding of only ADA:TNF, not ETN:TNF, to this FcgR with additional binding to undefined receptor(s). Despite subsequent increases in FcgRI and RIII later on, ADA:TNF still preferentially bound to FcgRII on the matured OCP with minimal binding to RIII, whereas ETN:TNF binding was observed only to FcgRI. Exposure of OCP to ADA:TNF:ZAP(toxin) complexes led to a significant reduction (4-fold) in mature OC due to complex internalization as compared to human IgG:ZAP+TNF conditions that was partially rescued only with the addition of FcgRII blocking antibody. Interestingly, a 1.5-fold reduction in mature OC was observed with ETN:TNF:ZAP.

Conclusion: Our in vitro findings demonstrate that human OCP can bind and internalize ADA:TNF complexes more efficiently than ETN:TNF complexes. In addition, this process is partially mediated through FcgRII. Clearance of the ADA:TNF complexes may help reduce exposure of the OCP to localized TNF by removing TNF more effectively in the joint environment. Additional in vivo analysis need to be done to verify these in vitro findings.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/adalimumabtnf-complexes-are-cleared-more-efficiently-by-human-osteoclasts-than-those-with-etanercept-through-fcg-receptor-binding-and-internalization/